Rack before 1.1.3, 1.2.x before 1.2.5, and 1.3.x before 1.3.6 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
Rack, a Ruby web server interface, is vulnerable to a denial-of-service (DoS) attack due to a flaw in how it handles form parameters. Attackers can exploit this vulnerability by sending a large number of crafted parameters, causing the server to consume excessive CPU resources and become unresponsive, effectively shutting down the application.
Step 1: Payload Generation: The attacker crafts a large number of form parameters. These parameters are designed to trigger hash collisions within Rack's internal data structures. This involves creating parameters that, when hashed by Rack's vulnerable hash function, produce the same hash value.
Step 2: Request Submission: The attacker sends an HTTP POST request to the vulnerable Rack application. The request body contains the crafted form parameters. The number of parameters is chosen to overwhelm the server's processing capabilities.
Step 3: Parameter Parsing: The Rack application receives the HTTP POST request and begins parsing the form parameters. The application uses its internal hash function to store the parameters in a hash table.
Step 4: Hash Collision Trigger: Due to the crafted parameters, the hash function generates the same hash value for many parameters. This causes all of these parameters to be placed in the same bucket within the hash table.
Step 5: CPU Exhaustion: When the application attempts to access or process the parameters, it must iterate through the bucket containing all the colliding parameters. This operation takes a significant amount of time, especially with a large number of colliding parameters. The CPU usage spikes, leading to denial of service.
The vulnerability stems from Rack's use of a hash function (likely a default Ruby hash) to store and retrieve form parameters. The hash function, prior to the patched versions, was susceptible to hash collisions. Attackers can craft a set of parameters that all hash to the same bucket within the hash table. When the server attempts to process these parameters, it spends a disproportionate amount of time resolving these collisions, leading to high CPU usage and a DoS condition. The root cause is the lack of a mechanism to prevent or mitigate the impact of hash collisions, allowing an attacker to control the hash function's behavior and degrade performance. This is not a buffer overflow or race condition, but rather a algorithmic complexity issue.